OMAPDSS: DISPC: use get_framedone_irq in disable_digit_out

dispc_mgr_disable_digit_out() needs to wait until the DIGIT output is
turned off. This is done with either VSYNC irq on OMAP2/3 and
FRAMEDONETV on OMAP4+. It currently uses a rather hacky way to decide
what irq to use.

This patch changes dispc_mgr_disable_digit_out to use
dispc_mgr_get_framedone_irq to find out if there's framedone irq on this
SoC, and if not, uses VSYNC.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>